The conversion of the dominant recycle stream (about 60% of the weight) from crushing lead acid batteries to a useful product has been costly, highly energy intensive and generates considerable hazardous dust, sulfur dioxide, carbon dioxide and carbon monoxide pollution when performed in smelters. The recovered lead metal from the grids is simply smelted while the lead sulfate/red lead/lead metal powder paste containing antimony and barium sulfate waste must be mixed with carbon and smelted in the furnaces along with the lead metal grid to achieve a practical and low cost route for handling the paste material. Barium sulfate is usually added at 0.5 to 2% levels in lead acid batteries to control lead sulfate crystal size during electrode charging/discharging. The smelting with carbon converts the lead sulfate to sulfur dioxide which must be scrubbed along with the extra lead dust and carbon powder which is generated. This is also a very energy intensive and polluting process. The lead metal from the smelters is purified by sparging and forming a flux, which may lead to further contamination resulting in further losses and costs. For the electrodes used in lead acid batteries, a form of litharge is produced by the slow oxidation of molten lead metal (99.999% purity) at 380-500° C. to form an impure form of a mix of litharge and fine lead metal particles at 15-30% by weight. It is too expensive and time consuming to try to oxidize this remaining lead to litharge. This process also forms massicot, which is not desired because of its slower reaction during electrode formation.
In U.S. Pat. No. 4,222,769 spent battery paste is desulfurized and then transformed into metallic lead by roasting in the presence of a carbon reducing agent.
U.S. Pat. No. 4,769,116 discloses treating exhausted lead acid battery paste with sodium hydroxide to produce a solution of sodium sulfate and a desulfurized paste which is subjected to electrowinning to produce metallic lead.
U.S. Patent Publication No. 2006/0239903 to Guerriero discloses high purity lead hydroxide and lead oxide from spent acid battery paste that has been desulfurized and converted into a carbonate or hydroxide and then calcinated at 500° C. to obtain pure PbO. The multi-step process includes subsequent treating with acetic acid. The lead acetate solution was treated with an alkali or alkaline earth hydroxide to produce lead hydroxide.
U.S. Patent Publication No. 2010/043600 to Martini discloses a process for recovery of high purity lead compounds from electrode paste slime. The process includes dissolving lead oxide in the paste in suitable acid, reducing any insoluble lead dioxide with hydrogen peroxide, a sulfite or sulfurous anhydride, converting the lead oxide to lead sulfate and then treating the lead sulfate in a solution containing an acetate, calcinating the desulfurized material to get impure lead monoxide followed by leaching of the lead monoxide with acetate acid followed by filtering and then treating further with an alkali hydroxide or alkaline earth hydroxide to obtain soluble acetates to get a precipitate of lead hydrate or lead monoxide.
U.S. Pat. No. 7,507,496 to Smith et al relates to the selective removal of sulfate from battery paste and recovering Pb3O4 which has small amounts of impurities and can be separated from the impurities by dissolution.
U.S. Pat. No. 5,211,818 discloses a process wherein the paste sludge resulting from the exhausted batteries is treated with a solution of ammonium sulfate and the metallic lead constituent is recovered by electrowinning.
International Publication No. WO99/44942 discloses a process of producing lead monoxide from spent lead batteries using fluxing agents and an organic reducer in the calcinations step at a temperature of 400° C.-450° C.
Typically litharge is made in the Barton process by heating lead metal to 380-500° C. to keep it molten and passing controlled amounts of air or oxygen through it to partially oxidize it to litharge containing up to 25% lead with strong agitation. The product lead oxide (litharge) that forms initially contains very fine lead metal (up to 25%) which is intimately mixed into the litharge lead oxide mix for the electrodes, which more slowly reacts during electrode plate formation. The litharge (leady oxide) contains up to 5% massicot (another allotrope of lead oxide).
Currently, litharge is purchased commercially by the lead acid battery industry as leady oxide (PbO plus about 15-75% lead metal powder) because the oxidative conversion of molten lead metal at about 380-500° C. in a Barton reactor or rotary reactor slows down as the lead metal level is reduced. Therefore, for economic reasons, 15-25% lead metal powder is left in the PbO to be reacted (oxidized) later during the electrode plate electrochemical formation. The plate electrochemical formation step requires up to two days with the positive electrode requiring the most lengthy times for formation. Presently, the leady oxide is used for both the positive and negative plates and is also highly reactive when mixed with sulfuric acid prior to actual plate formation. A desirable material for the positive plate electrode is Pb3O4 (e.g., red lead). However, Pb3O4 is too expensive for commercial use in making batteries, and the Pb3O4 material available commercially is only about 25% Pb3O4 with the balance being PbO present as both white massicot (beta form) and orange litharge (alpha form).
Massicot does not convert readily, if at all, to red lead when heated in the 450-500° C. range and may inhibit the conversion of litharge to red lead. This accounts for the lower purity of the commercial Pb3O4 since the reaction is primarily on the surface which gives good color (orange-red), but the further oxidation of the particle interior of the PbO is slowed or stopped by the presence of massicot. The presence of massicot in the litharge also slows down the electrochemical formation production step for the lead paste electrodes during formation whether for the positive or negative electrodes along with the extent of total formation capacity being reduced initially in the battery electrodes.
Thus, there is a need for a process which produces high purity litharge (preferably >95% purity) more economically and quickly with no massicot present. Such high purity litharge offers substantial savings in electrode formation production time, better quality and also allows for further oxidation to higher purity red lead. Such ease of oxidation of litharge to red lead significantly reduces the formation time of the electrodes of a lead acid battery by 50% or more.